Throughout the years, electrolytic cells have been made in a wide variety of shapes and sizes, including concentric, cylindrically shaped electrolytic cells. In fact, some of the earliest electrolytic cell designs were concentric, cylindrically shaped cells (see for example U.S Pat. Nos.: 522,617; 590,826; 673,452; 914,856; 1,074,549; 2,583,101; 3,812,026; 3,984,303; 4,117,116; 4,256,554; 4,374,014; and B388,701.
In concentric, cylindrically shaped cells, one electrode surrounds the other electrode. In the various patents of the prior art, the anode and the cathode have each occupied both the inner position and the outer position. Most commonly, however, the cathode surrounds the anode and is separated therefrom by a hydraulically permeable diaphragm. In chlor-alkali cells, sodium chloride brine solution is fed into an anode compartment where it is electrolyzed to form chlorine. Chlorine forms large bubbles and rises to the top of the anode compartment where it separates from the brine and is removed. During operation of the cell, a portion of the brine flows from the anode compartment, through the hydraulically permeable diaphragm, and into the cathode compartment. There, it is electrolyzed to form hydrogen and sodium hydroxide. Hydrogen forms small bubbles and is swept away from the diaphragm and the cathode by additional brine flowing through the diaphragm into the cathode compartment. The hydrogen gas bubbles flow into an upper portion of the cathode compartment, where they are separated from the sodium hydroxide/brine mixture.
Phenomena of bubble formation is discussed in U.S. Pat. No. 4,265,719 "Electrolysis of Aqueous Solutions of Alkali Metal Halides Employing a Flexible Polymeric Hydraulically Impermeable Membrane Disposed Against A Roughened Surface Cathode; and U.S. Pat. No. 4,329,218 "Vertical Cathode Pocket Assembly for Membrane Type Electrolytic Cell", Sorenson, Ezzell and Pimlott. These patents are incorporated by reference for the purposes of their teachings about hydrogen bubble formation at cathodes in chlor-alkali cells.
With the recent advent of ion permeable membranes which are used to replace hydraulically permeable diaphragms, the use of cylindrically shaped, concentric, electrolytic cells has declined, and particularly their use for the production of chlorine and caustic. Since ion permeable membranes do not allow substantial amounts of free water to pass from the anode compartment into the cathode compartment, there is nothing to sweep away the hydrogen bubbles. As a result, hydrogen builds up and tends to block the flow of electrical energy at electrodes, thus increasing the amount of energy the cell uses. This blinding problem is present in most electrolytic cells that produce a gaseous product at one of the electrodes.
Another problem with cells that use ion permeable membranes is the somewhat short lifetime of the ion permeable membrane in some cells. Short membrane lifetime is sometimes a particularly troublesome problem when composite membranes (2 or more layers laminated together) are used. Such membranes are prone to delamination, which almost totally ruins the usefulness of the membrane. Membrane delamination is thought to be caused by exposure to highly concentrated alkaline hydroxide solutions or by simultaneous exposure to two phases, a liquid and a gaseous phase. An electrolytic method which lengthened the lifetime of composite membranes would certainly be desirable.
The present invention provides a method for operating a cylindrically shaped, electrolytic cell employing ion permeable membranes in a manner to minimize the build-up of gas at the cathode, thus minimizes electrical inefficiencies due to gas blinding. In addition, the present invention provides a method for operating an electrochemical cell in a manner to minimize delamination of composite ion permeable membranes. The present invention is particularly useful in chlor-alkali cells.